Radar system with improved area type moving target indicator



B. C\UTLER July 20, 1965 RADAR SYSTEM WITH IMPROVED ARE'A TYPE MOVING TARGET INDICATOR Filed June 14, 1960 3 Sheets-Sheet l July 20, 1965 B. CUTLER RADAR SYSTEM WITl-I IMPROVED AREA TYPE MOVING TARGET INDICATOR Filed June 14, 1960 5 Sheets-Sheet 2 Jul? 20, 1 B. CUTLER 3196434 RADAR SYSTEM WITH IMPROVED AREA TYPE MOVING TARGET INDICA'IOR F'iled Jun 14, 1950 5 Sheets-Sheet 3 United States Patent O 3196,434 RADAR SYSTEM WITH IMPRUVED AREA TY?E MU'VENG TARGET INDE'CATGR Eurton finden, Los Angeles, Caiif., assignor, iay mesne assignments, to internationai Telepiwne and Telegraph Corporation, New York, N.Y. a corrpoxation of Maryiand Filet] (Inne 14, 1960, Sex. No. 336,611 143 Ciaims. (C1. 343--7.'7)

This invention relates to radar System and more particularly to radar systems of the moving target indicator type.

In the prior art, rnoving target indicating systerns have been of two generic types, the coherent type MTI (moving target indicator) and the non-coherent type. The general purpose of any MTI system, whether coherent or non-coherent is the eflective presentation of signais corresponding to moving targets to the exclusion cf, or With discrimination against targets which do not rnove. Tims, large land masses such as mountains and other fixed objects which give radar reflection (i.e signal returns) to ordinary radar sets may be discriminated against in the moving target indicator radar system, thereby reducing confusing information and aifording other distinct and Wel1 known advantages to the radar operator. Considerable prior art description establishing the background and relating to MTI radar systen1 in general appears in Chapter 16 of the Radiation Laboratory Series, V01. l, (First Edition, third impression), published in 1947 by McGraw-I-Iill Bock Company of New York.

The instant invention concerns Area MTI which is a f01n1 of non-coherent MTI system. The use of the terrn Area is based on the concept of fixed target cancellation on the basis of a Video comparison over a period of time spanning many radar transmitted pulses; whereas coherent systems as explained in the above mentioned V01. 1, Chapter 16, cf the Radiation Laboratory Series, effect an instantaneous type of signal cancellation (i.e., cancellation between echoes from one repetition interval to the next).

Although a more complete discussion of the general concepts f Area MTI System and their relation to the instant invention will be undertaken below, it may be stated here in general that prior art Area MTI systems suffer from two significant disadvantages. First, it is pointed out that whenever cancellation is effected by direct comparison of video waveforms, it is obvious that good cancellation depends on good bandwidth in all components of the system including good resolution in the storage tube. Any deterioration of waveforms caused by delays, phase distortion, etc., tends to result in generation cf vestigial video signals in lieu of complete cancellation.

The second disadvantage of prior art MTI (including prior art Area MTI) stems from the fact that the subclutter visibility in an A1ea MTI system is very much dependent upon the amplitude dynamic range (i.e. output response as a function of input signal) in the system as a whole. Since large block signals particularly at close range tend to give saturation Signals to the radar receiver, the presence of a rnoving target at a range coincidental with such a fixed object tends to be obscured since there is no dynamic range rernaining in the receiving system to accornrnodate the signal return from the said moving target. The use of variable receiver sensitivity as a function of range, sometimes called STC (Sensitivity Time Constant) does not eliminate this problem since large fixed targets and small moving targets at comparable ranges may reflect vastly different amounts of energy.

In consideration of these disadvantages of the prior art, the principal objects of the instant invention follow:

It is an object of this invention to provide an area MTI systen1 which has improved sub-clutter visibility.

lt is a further object of this invention to provide an area MTI system which requires less system bandwidth and storage tube resolution than that required by prior art area MTI systems.

It is a further object of this invention to provide MTI cancellation in an Area MTI system which is substantially free of vestigial video, thereby producing more efiective cancellation than heretofore obtainable.

It i also an object of the present invention to produco an Area MTI systern which dass not require a high order of systern stability, thereby facilitating simple attachment to existing radar systems.

It is yet another object of the present invention to produce an Area MTI system which is n0t critically dependent upon specific radar system arameters for adequate erformance.

Other objects and achievements of this invention will be apparent as the further description and claims are studied.

FIGURE 1 show a simplified block diagram of a typical Area MTI system of a prior art type suitable for the GCA application.

FIGURE 2 illustrates a system for implementing an improved Area MTI according to the present invention.

FIGURE 3(a) illustrates a typical video waveform as may be expected from detection of the output of a radar receiver I.F. stn'p showing typical fixed target ulses mingled with a background of noise.

FIGURE 3(b) shows a typical fixed target pattern as might be present as a visible or charge pattern on a storage tuoe of sorne suitable type as associated with the present invention.

FIGURE 3(c) shows a video waveform sirnilar to FIGURE 3 except that fixed ta1get pulses have been substantially removed in accordance With the present invention.

FIGURE 4 is a block diagram showing a means of advancing and elongating an AGC signal in respect to a corresponding target signal.

Although it should be made clear that the instant invention is readily applicable to any scanning radar, as is also fundamentally the case With most MTI systems, the discussion following Will be directed toward one particu- 1ar scanning radar, namely, GCA (Ground Controlled Approach). This is done because GCA is a weil known scanning radar equipment in which the need for MTI is We1l recognized, and for Which the improved Area MTI in accordance With the present invention is particularly advantageous. For the readers information, U.S. Patent N0. 2555101, issued February 25, 1944, to Alveraz is an excellent reference describing the elements 0f a GCA system (without MTI of any type).

Before embarking on a detailed description 0f the present invention, it is thougnt to be desirable to discuss the prior art and the enviromnent of the invention in Same detai1.

The GCA (Ground Controlled Approach) problem arises because of the need for controlling the flight of aircraft during periods of reduced visibility, specifically during the approach and landing. The function of the GCA equiprnent is to aid in the establishment of a precision g1ide path in the final phases of approach and 0f landing of aircraft. This may be Weil accomplished by means of a highly accurate (Ground Controlled Approach) radar, in conjunction With a communications link between the ground equiprnent and the aircraft.

One of the most serious problerns associated with GCA precision landing is that cf ground clutter. The smaller the etfective radar target area of the approaching aircraft, the more severe the ground clutter problem. This eflect is emphasized because the aircraft is seen virtually headon during GCA operations, so that the radar receives echo power corresponding to the minirnum echoing area of the plane. In the past, one technique for reducing the visible elfects of clutter has been to reduce system gain in the final approach. This technique is based on the echoing area of the target being comparable to the echoing area of the clutter. However, as planes become more advanced in design, and echoing areas tend to fall consisteritly as 10W as 0.2 to about 2 square meters, headon, this particular technique is not particularly useful. Furthermore, the echoing area of the plane is not constant. Polar patterns taken of various types of aircraft indicate that echo signal strength under these conditions must be expected to vary 30 db to 40 db, thereby producing sigma] peaks and nulls during any given approach. For propeller-driven aircraft, the nulls tend to be filled-in somewhat by propeller modulation; however, for jet aircraft the beneficial etfect of this prop modulation is absent. From this discussion one realizes the need to handle the clutter problern directly for the GCA applications, by reducing the clutter signal return relative to the signal return. For some sites an MTI range on the Order of 3 n'1ileS from touchdown is adequate, however, many sites present a sutficiently severe problem to dictate GCA MTI ranges as rnuch as 6 to 10 miles. The areas of solution to this problem are discussed in the following. Although Coherent MTI systems as described in U.S. Patent 2717377 can and have been used in GCA equipment, the added complication and the lower order of resultant stability and r'eliability are significantly disadventageous, however.

The GCA clutter problem is related to the nature of the site and is frequently aggravated by the existence of antenna side lobes. The types f antennas necessitated by the rapid scan operation do not lend themselves to the ordinary techniques for efi"ecting carefully controlled side lobe characteristics Having realized that some form of MTI is required to handle the clutter problem, the question arises as to the most practical form that this MTI should take for radar systems such aS GCA. Since many hits per beamwidth are desired and only moderate ranges are required, high scanning rates and high pulse repetition rates (i.e., high data rates) naturally follow.

Any MTI system chosen for the GCA application presents its own unique design roblems. The coherent type (line-by-line) MTI referred to previously, although capable of good sub-clutter visibility and general perforrnance poses great technical complication of design and consequently is an expensive MTI. Great care is necessary to obtain the necessary stability of the radar transmitter, receiver local oscillator, and the various video and pulse handling circuits. Klystron and other local oscillators tend to exhibit both short and long terrn instabilities, and the clutter instability problem due to the high scanning rates and the limited number 0f hits per beamwidth become difiicult problems in the coherent MTI application.

Furthermore, and often of prirnary importance, there is the matter of blind speeds, a pherxomenon peculiar to line-by-line MTI systerns. (See See. 16.9 of Radar System Engineering, by Ridenour, volume 1, Radiation Laboratory Series, published by McGraw-Hill, New York, New York.) It is often diflicult to reconcile blind speeds With a variety of landing speeds Where the MTI radar must handle a variety of approaching aircraft ranging from light planes to jet fighters.

It Will be noted that Area MTI (which is not a line-byline system) is not only capable of passing moving targets exhibiting diverse velocities, but Will do so without significant reduction in visibility. Furthermore, Area MTI, which is a non-coherent MTI system, has the intrinsic capability of clutter gating to maintain maximurn vis= bility Of a moving target in the clear. Coherent systerns, on the other band, inherently deteriorate targets in the clear by sorne irreducible minimum amount.

In view of these considerations, Area MTI may be considered to be a satisfactory technique for use with GCA as well as the general radar case. After the following discussion of prior art Area MTI system, the background of. the instant invention Will be better understood.

The principle of Area MTI as exemplified in one prior art equiprnent involves the utilization of the properties of the barrier grid storage tube as a differencing device. The mechanisrn includes recording, over a period of several radar scans, a charge pattern on the dielectric surface of the storage tube which is a replica 0f the particular area being scanned by the radar. Moving targets are resolved, then, by virtue of their spatial change in position while fixed radar return is cancelled after equilibrium is reached due to the lack of difierence inforrnation on successive scans. The charge pattern may resernble the radar indicator ORT display since the storage tube is swept in a similar manner.

This particlar prior art Area MTI system is delineated in considerable detail in the final engineering report entitled Area MTI System f0r Radar Set AN/FPN-l6, prepared for the Department of the Air Force under Contract N0. AF 30(602)-784 by the Radio Corporation of America, Moorestown, New Jersey. A copy of this unclassified report is currently available from the Arrned Services Technical Information Agency, Document Service Center, Knott Building, Dayton 2, Ohio.

The radar system shown in FIGURE 1 serves to illustrate generally certain cornponents of a typical GCA system in which Area MTI is incorporated. The partieular Area MTI system illustrated in FIGURE 1 is the basic system described in the above referenced final engineering report and as previously mentioned, uses a barrier grid storage tube as a differencing device. FIGURE 1 shows the conventional azimuth antenna 101 and elevation antenna 102 which, on a time sharing basis, scan the approach zone to an aircraft landing field for purposes of transmitting pulsed energy and receiving resultant echoes from aircraft in the approach zone. For that purpose, pulsed energy from the transmitter 103 is transferred through the radio frequency azimuth and elevation (Az-El) switch 104 to antenna 101 and antenna 102 alternately. For the purpose of initiating energy pulses in timed relationship, synchronizing pulses are transferred to transrnitter 103 from the synchronizer 105.

The resulting echoes (reflected signals) received on the azimuth antenna 101 or elevation antenna 102, as the case may be at any one particular time, are transferred through the conventional transmit-receive (TR) switch 106 to the crystal mixer 107 of a superheterodyne type of radar receiver, such crystal serving to transform the received echo signals to a predetermined intermediate frequency, as is well known in the art. Such signals of intermediate frequency are in conventional manner arnplified in the preamplifier 108 and are further amplified, then amplitude detected by the intermediate frequency (IP) amplifier and detctor 109, The video output of detector 109 is amplified to a predetermined value by video amplifier 110. Up to this point the transmit-receive system briefiy described is typical cf the average GCA system and is conventional in all respects. As a matter of fact, using only one antenna and deleting the Az-El switch 104, the transmit-receive system to this point is typical of most surveillance type radar systerns.

Continuing the description of FIGURE l, in .a radar system not employing any version of an MTI system, the output of the video amplifier 110 is fed to a display indicating system wherein a cathode ray tube is used in displaying t-he Video signals. In the prior art Area MTI system depicted in FIGURE 1, the video frorn the video amplifier 110 is first routed to the storage tube canceller 111 instead cf the display system. After due processing within the storage tube, tl1e video is fed to the display system via the output amplifier 112.

The storage tube -canceller 111 includes a barrier-grid storage tube of the Radiohon type and an R-F signal separation circuit permitting simultaneous writing 0f the video input signal and reading of the desired output difference signal. This R-F signal separation teohnique, as adapted to the Radichon instrumentation, is explained in =detail in the RCA Review (a technical journal), volume VI, published Inne 1955 by the Radio Corporation of America, Princeton, New Jersey. The R-F amplifier and detect-or 112 w-hich receives the R-F diiference signal frorn U18 storage tube canceller 111, consists of a tuned R-F .arnplifier, responsive to the frequency of the R F oscillator used in Ihe signal separation circuit and an an1plitude detector. The output of the R-F amplifier and detector 112 is fed -to the indicating systern via line 113. Thus, video diiferencing is effected continuously as new informati-on is being made available to Ihe st-orage tube. Any difference between this new video and that already stored, is -the moving target information and is extracted via this radio frequency signal sepa1ation technique and is converted to video by Ehe detector 112. Synchronizer 105, beside supplying triggers to the transmitter, triggers the azimuth and elevation sweep arnpl-ifier 114 at a predetermined rate wherein Iange gates and sweeps are generated and sent to the deflecting electr-odes of the storage tube canceller. The azim=uth and elevation angle data generator 115 provides analog azirnuth and elevation antenna scanning information on a tirne shared basis. This angle information is used to modulate horizontal and vertical components of ihe storage tube sweeps such (hat scan of the storag tube surface resernbling the cathode ray tube scan in the indicator is ellected. More specifically, the sweep cornponents thus generated in sweep amplifier 114 provide Ihe storage tube Witl1 a type B scan whose sweep in the fange :cornponent direction is synchronized Wll3ll tl1e radar re-petition frequency and w-hose azimuth and elevation sweep is instantaneously synchronized wit-h the angular position of the corresponding scanning antenna beam.

In the above type of Area MTI operation, only the diiference signal is thus observed at the output of tl1@ R-F arnplifier and detector 112, i.e., a target whioh is continuously changing in time phase auch as a rnoving target. T-l1e barrier grid storage tube used in this particular type 0f Area MTI system thus effectively has the ability (in eifect) to cancel targets of constant amplitude and phase (position in range and angle) internally.

The internal signal cancellation process in t-he barrier grid storage tube is explained in ehe aforesaid reference (Area MTI System for Radar Set AN/FPN-l6"). A brief summary of this phenomenon follows, in order that the reader may have a clear unclerstanding of the prior Area MTI systern just discussed, and thus be prepared to fully appreciate the difference and the advantages of the present invention.

When a signal is impressed upon the plate cf a scanned barrier grid storage tube, the beam deposits on the insulating target a charge pattern, varying in intensity, in accordance with the arnplitude variations of the Signal. If the surface is again scanned over Ehe sarne path With no signal impressed, the bearn Will ren1ove the charge pattern, f[l1us reading oft" a signal Which is in polarity a mirror image -of the original sigma]. Both during the writing and the reading, the signal will appear on Ihe colleotor as a modulation of the secondary bearn. In this operation, the tube has acted as a mernory device, storing and subsequently reprodncing a signal. If, however, Ihe same signal is imp1essed upon the tube on each successive scan,

the beam will already have deposited tl1e charge pattern necessary to match this signal variation. Therefore, that area under the beam at any given time is instantaneously at equilibrium potential. N0 charge will be deposited 011 the target 011 succeeding scans, so that the secondary bearn will be constant and unrnodulated. Thus signal will appear on the collector unless a variation in the input signal requires deposition of charge by Lhe bearn.

The above type of Area MTI is based on the ability of the particular type storage tube to act as a combined mernory and subtraotion device whereby stationary targets being of constant phase and arnplitude a1e cancelled and during successive scans only the difierence appears at the collector of the storage tube, said difference being that farget information changed in phase and/or amplitude since the previous scan such as an aircraft or moving vel1icle.

Several practical limitations exist With respect to this prior art type of Area MTI systern because tl1e maxirnum resolution capability of the storage tube restricts the Area MTI to a limited portion of the scanned radar area. Since practical storage tubes have a limited number 0f resolution elements in any direction across the storage surface, the greater the range which is accommodated by the storage area, t-he lower the nurnber of resolution elements per unit of range. Consequently, prior art Area MTI systems, in view of their need for good videofidelity, are unable to obtain good erformance over lang range intervals. The cancellation of a fixed target return is also lim-ited fundamentally' in prior art Area MTI by the finite Spot size of the storage tube electron beam since this too relates to the fidelity with which charge pattern storage can be eliected. In this particular Area MTI systern discussed in the prior art report noted above, th8 MTI sector was intentionally limited by a range gating scherne to a range 01 three miles.

-I-t Will be realized from t-he foregoing discussion, that existing (prior .art) Area MTI systems employing storage tubes are limited, due to the resolution capabilities of storage tubes.

In view of the above, an Area MTI system, according to the present invention, utilizing the properties of a storage Iube without sufiering Ehe resolution limitation of the prior art, constitutes an important advance over -the prior art. As an example, relative to the previously mentioned GCA system, a 10 mile precision Area MTI systern is readily obtainable using storage devices capable of only a relatively 10W order of resolution.

Referring now -to FIGURE 2, an MTI systern in accordance With the instant invention is shown. The transmitter-modulator 103, trigger generator 105, TR 106, crysta1 mixes 107, IP preamplifier 108, IF amplifier and detector 109 and video arnplifier 110 are Iypical for an average system configuration of a normal raclar set, and Will be recognized as a duplication in part of FIG- URE 1, except for the accommodation of a signal from 213, 10 be explained later. Antenna 205, of .conrse, is also typical of the average microwave antenna array, such as the azimnth or elevation antenna array referred to in FIGURE 1, 101 and 102, respectively.

lt will be noted upon close examination of FIGURE 2 that a feedback path from Video output 206 reaches tl1e IF amplifier 109 via threshold clipper 201, amplifier 202, sto1age tube 204, filter 211 and detector 213. These five individually simple units, plus writing and reading sweep generators 203 and 212, When incorporated into a normal radar set, are capable of providing the radar set Witl1 Area MTI type operation by attachment, and modification of IP amplifier and detector 109. This unique confimration is capable of automatically regulating the gain of the IF stages -0f a radar receiver at a Video rate and in accordance With selected targets, as Will be explained. A key fast relating t0 the function of the present invention as an MTI system is the fact that the IF amplifier is substantially only afiectecl by targets having a comstant ange relationship over a period of tirne spanning a number of radar transmitted pulses, i.e., the controlled gain of the IF amplifier is a function of the return amplitude 0f targets of constant range. A charging or learning tiine constant of at least several scan cycles is relied upon t0 substantially prevent the storage of targets which change position (i.e., move) between scans. If a moving target (aircraft) is Within the perimeter of a fixed target, then the incident energy received from the moving target will add to tl1e incident energy from the ground target at the range of coincidence. Therefore, with the reduction of gain thus imposed by gronnd target feedback cf stored Video during that interval of time in which the ground target is received, a rnoving target within said perirneter will be able to stand above the ground sigma! because it represents refiected energy in excess of tn-e corresponding stored signal used for gain control purposes. The receiving system, moreover, will obviously never be saturated by massive fixed targets, tlins eliminating one of the principle factors limiting su'o-cluiter visibility in any MIT system.

Referring again to the drawings, a normal radar systern video pulse train at output 236 fo1 an arbitrary direction of scan n1ay be seen in FIGURE 3(a). In tl1is example of a typical range period, only fixed ta1gets are shown. lt is assumed at tl1is time that the Area MTI section 2537 Within the double lin-es forming a black Box addition to a radar set is not present. Fixed targets 301 and 333 are at saturation level and fixed target 382 is another large, but not saturating, target. Any moving target within the perirneter of the ground targets, represented by the signals 391, 3%2, and 393 in FIGURE 3(a), will not be discernible. The stationary targets being at or near maximum (saturation) amplitude, tend to swamp out the moving target when it is within the perimeter of any of tlze said siationary targets as viewed on a .PPI presentation.

New assu1ne tl1at the Area MIT section 237 is incorporated into the otherwise relatively simple radar set depicted in FIGURE 2. The video outpnt 2G6 is now aslo routed to the threshold clipper 291 via line 2%. The threshold clipper 2fiil acts as a biased clipper which. passes video above level 313 and not that between 3434 and 313. Tl1us, only (hat level which exceeds the new tln'esh0ld level will be stored by storage tube The output of the threshold clipper 201, the base level of which is now as illustrated by dotted line 333, is fed to ampliiier The gain setting of amplifier 232 determines 110W xnuch of. the video above this threshold level will be sto1ed on any given scan. This is because the sto1age surface response, like that of any cathode ray device, reqni1es finite build-up time, therefore, recording er charging signal amplitude produces an ellective storage build up tiine constantly inversely proportional to said recording or charging signal amplitude, It will be seen that storage tube 234 will then rnore er less gradually build up and retain a 1nosaic of all fixed targets 215 it is continuously supplied with video data via amplifier 2432. FIGURE 3 (b) is typical of the storage tube output appearing on the AGC signal line 2%9. The AGC pulses 305, 366, and 3f97, corresponding to signals 301, 302, and 303 are cf negative polarity to control the gain of the IF amplifier 1%)9, assuming that an IF amplifier adaptable to pulsed gain control, as for example, the IF amplifier shown in U.S. Patent 2,751587 is used at 1&9. Tlie base or quies- Cent level 303 is such that during the interval between control pulses, the IF arnplifier is at nominal full gain. The time interval between the first application of any signal input to the storage tube 204 and the growth of a signal output at 239 suiiicient to control the gain of the IF arnplifier 169 is, in view of the storage tube charging (learning) time constant discussed above, dependent upon the gain setting of amplifier 202. This interval of ti1ne in a representative case, will be about 5 to 30 seconds, depending 0n the said gain setting of the amplifier 202,

controlling the input to the storage tube. It is of great impertance that, due to this 5 t0 30 second learning delay, n1oving targets will not persist in any one spot long enough to build up a charge pattern on the target surface of the storage tube. Therefore, no AGC signal corresponcling (o moving targets such as 311, will be derived from the storage tube 294 and no substantial efiect on moving targets will be evident at -output 206. On t'ne other band, a video target, the range of which is approximately constant froxn one scan period to the next, will coniinuously add to the original chargc pattern on the target surface of the storage tube and will, during the learning period, builcl up a charge pattern whereby the charge is suii'icient to obtain a Signal output that will afiect the gain of the IP amplifier. Oncc a gain control signal, such as 365, begins to build up, the corresponding signal 391 be ins to decline in amplitude. The build-up impetus into 234 is thus reduced, and a degenerative cycle begins and results in the establishment of an equilibriurn condition with respect to any given signal in the manner to be expected in any inverse feedback arrangement. Note that in FIGURE 3(c) residual Signals 312, 309, and 313 corresponding respectively to 301, 392, and 303 are shown below the average grass level.

It will be noted that two moving target signals, 311 and 314, are shown in FIGURE 3. Target 311 does not show in FIGURE 3(a) because it is completely enveloped within the saturated fixed signal 331. On FIGURE 3(c) however, the moving target 311 rides above 512 in accordance with tl1e previously explained operational principles of the prcsent invention. Target 3.14, being in the clear, is substantially the same in FIGURE 3(c) as in 3(a).

The amplitude of residual signals 309, 310, and 312 is illustrated f0r an arbitrary setting of the video threshold bias represented by 313. Obviously, the higher the 313 level is sei (less video passed to storage), tl1e higher Ehe 39, 310, and 312 residual signals will be in FIGURE 3(c). Convcrsely, an adjustment of 313 close to the zero reference Video level 394 adrnits more noise t0 storage and therefore, some empirically determined cornpromise in the adjustment of this threshold will be necessary.

It is of considerable importance that the leading edge and trailing edges of the AGC signals derived froxn the Mmrage tube 2M output and shown in FIGURE 3(b) tend to bracket the corresponding Video signals shown in FIGURE 3(a) This phenomenon is due to a certain arnount of blur 011 die stored signal edges. In any storage tube, the contrast of the display depends upon the charao teristics of the target surface and the pertinent electron optics. In the present invention, some blur of a stored Signal will make tl1e ontput of the storage tube corresponding to any stored signal block appear at read-out t0 begin slightly ahead of the corresponding leading edge of the read-in signal and terminate correspondingiy late. Thus, inherent limitations in respect to storage tube surfaee granularity and in tne sharpness of focus of the clectron beam are actually of benefit in the arrangement of the present invention, since thereby vestigial signals due to slight inherent delays and circuit infidelities are eliminated.

Concerning the choice of a storage tube for the present invention, it has been determined that the designer has considerable choice. A handbook, entitled Sto1agc Tubes and Their Basic Principles, by Knoll and Kazan, published by John Wiley and Sons, Inc., New York, New York, discusscs in considerable detail various types of storage tubes. In this publication, several types of storage tubes capable of simultaneous reading and writing are discussed in pages 53 through 61. Included among the vari- 0us types discussed is the graphechon type storage tube which is likely t0 be the preferred type for n10st variations of the instrumentation of the present invention.

As has been previously explained, thc storage tube target surface is scanned mach in the sarne manner as the surface of the typical cathode ray tube used in radar indicating systems, i.e., the storage tube is provided With angular and range components, wherein the range sweep is synchronizecl With the radar repetition frequency and the azimuth and/ o1 elevation angular sweep is instantaneously and continuously the analog of the angular position f the scanning antenna. In the oase of a dual gun storage tube, such as the graplxechon, each gun must be provided With such sweep components. In the present invention the sweep to the writing and reading guns is provided by the writing sweep generator 203 and reading sweep generator 212 (See FIGURE 2). Sweep generators 23 and 212 are triggered by the System trigger generator 1l5, thus the Start of the range sweep is synchronizecl With the radar repetition frequency. The angle sweep analog to control each sweep generator is derived from the antenna angle data generator 115.

When a storage tube is employed for writing and reading simultaneously, as in the case of the present invention, it is necessary to prevent the writing beam current modulation from generating a signal in the output. This is accomplished by intensity modulating the reading bearn at a frequency well above the maxirnum frequency contained in the writing signal.

In the instant invention, the reading beam is intensity modulated by R-F oscillator 210, the frequency of which is, for example, approxirnately megacycles. As a result, the desired output appears as amplitude modulated 30 megacycle Signal which is passed by filter 211. The lower frequency cornponents produc-ed by the writing beam are attenuated by the action of the said filter 211. This 30 megacycle amplitude modulated signal, the reading signal, is next fed to detector 213. This R-F Signal Separation scheme is a technique described in the literature for conveniently extracting an output Signal Without cross-talk from simultaneous writing. The detectecl output of the detector 213 is the desired reading signal, which is then ready to be fed in proper polarity to IF amplifier 189 as an AGC Signal via line 299.

In FIGURE 4, a variation in the circuitry for forming the AGC Signal on line 209 is shown. Depending upon the range factor per unit area over the storage tube surface, the blur eflect discussed herein, may or may not be suldcient to produce the desired leading and trailing edge overlap of received signals by the AGC signals 01 FIGURE 3(b) while maintaining adequate definition. The variation shown in FIGURE 4 amounts to alternative means included in the Area MTI Section 2117 whereby the AGC Signal could be macle to bracket the ground signal at the IF amplifier, thereby insuring that the leading and trailing edge of each ground Signal or block of ground signals do not appear at the output of the IF amplifier and detector 19. The range trigger delivered by the system trigger generator 15 to the writing sweep generator is delayed a relatively small predetermined amoi1nt of time by delay Eine 401 which can be a manually comtrolled variable delay for greatest flexibility. The effect of delaying the writing sweep is obviously to make the targets written appear advanced With respect to the reading sweep. With the leading edge of the AGC Signal train 308 now appearing at the IF amplifier slightly in advance of the corresponding ground Signal and having a duration of approximately that of the ground target Signal entering the IP amplifier, the trailing edge of a ground target Signal tends to fall outside the corresponding AGC Signal Which would terminate before the termination of the ground target Signal. Therefore, additional 1neans must also oe proided to increase the duration 0f the AGC Signal so that the said Signal can completel3'z bracket the ground target signal. This is accomplisl1ed by the mixer 4%, worl ing in conjunction with delay line 4-02, in series With the AGC Signal line 209. The mixer generally referred to as an er circuit in logic design, is basically a buffer or mixing circuit, which permits a nurnber of gate sources of common polarity to be connected to a common load, in this case the IF amplifier. The AGC Signal is 1nade to enter two inputs 01 the mixer 43. One inpnt is delayed a predetermined amount of tirne by delay line ll2. Therefore, after the termination of the AGC Signal at the undelayed input, the delayed Signal input continues to appear at the delayed input, and thus it is possible to completely bracket any ground target Signal appearing at the i a1npli1'ier with a conesponding AGC signal.

The application of pulsed gain control, per se, is well known and well described in the literature. A very satisfactory method of applying an AGC signal to an IP amplifier stage is delineated in considerable detail in U.S. Patent 2751,587 issued Inne 19, 1956, entitled Range Gated Automatic Gain Control. The circuit configuration shown in FIGURE 4 of the said patent could be incorporated into an IF amplifier strip, resulting in a satisfactory structure for 1199. The AGC signal in this particular AGC circuit (Patent 2751587) wonld oe fed to the range gated video input terminal 178A.

It sl1ould be pointed out that there iS considerable variation possible in the specific instrumentation of the storage medium. In place oft a dual bearn storage tube such as a graphechon, a cathode ray tube and vidicon carnera tu'oe comoination With optical link such as described in U.S. patent application Serial Nurnber 593,011, filed Inne 15, 1956, now U.S Patent N0. 2972,141 could be substituted. In said System, the oharging or learning time constant is achieved both through the build-up characteristic of a conventional long persistence cathode ray tube phosphor screen and the special build-up characteristic afiorcied by the vidicon camera tube when operated at low Space currents, as described. Of course, the writing sweep then would be applied to the cathode ray tube and the reading sweep to the camera tube.

Various other modifications and variations falling within the scope and concepts of the present invention will suggest themselves to tl1ose skilled in the art, accordingly, it is not intended tl1at the scope of this invention be limited to the implementations shown, the description and drawings being intended to be illustrative only.

What is claimed is:

1. A system of the character described comprising the combination of: a scanning radar systern including a radar pulse receiver the gain of which is controllable over the duration of, and in response to gain control pulses and having a video output; a selection circuit operating on said Videooutput t0 produce selected signals by passing only signal arnplitudes in excess of a predetermined threshold value; storage means scanned synchronously Wll.h said scanning radar System to store said selected signals, storage control means interposed between said seleotion circuit and said storage means for controlling the response rate of said storage rneans, said storage control means including n1eans to alter the rate of registry of said selected signals in said storage means so that more than one successive scan interval of said scanning radar systern is required to efiect registry of said selected signals in said storage means; and pulse translating means connected to said storage means to transrnit gain control pulses to said receiver in accordance with said selected signals stored in said storage means.

2. A System of the character described comprising the cornbination of: a scanning rada1 System including a radar pulse receiver the gain cf which is controllable over the duration of and in response to gain control pulses and having a Video output; a selection circuit operating on said Video output to produce Selected signals by passing only Signal amplitudes in excess of a predetermined value; cathode-ray type storage means scanned synchronously with said scanning radar System and adapted to store said selected signals; storage control means interposed after said selection circuit for controlling the response rate of said storage means, said storage control means comprising the inherent writing Signal integration characteristic of said Storage means, thereby producing an effective Writing tin1e constant to control the rate of registry of said selected signals in said storage means so that rnre than one successlve scan interval of -said scanning radar system is required to produce sufllcient registry of said selected signals in said storage means to permit the derivation of corresponding read-out signals; anal pulse transmission rneans connected to said storage means and adapted to transmit said read-out signals as gain control ulses to said receiver in accordance with the degree of registry of said selected signals storecl in said storage means.

3. A movlng target indicating radar systern comprising -the combination cf: a radar pulse receiver the gain of which is controllable over tl1e duration of and in response to gain control pulses, said receiver having a Video signal output; a storage rneans for accepting und storing said Video signal output, said storage means requiring a plurality of video groups corresponding to a plurality of scan cycles to obtain substantial signal storage, said storage means therefore being relatively insensitive to any ortion 0: said video output corresponding to a moving target echo; and pulse translating means connected to seid storage rneans and adapted to transmit gain control pulses to said receiver in accordance with stored signals fron1 seid storage means.

4. An area moving target indicating radar systenx comprising the combination of: bearn radiating means for radiating a beam 0f electromagnetic energy into space anal receiving electromagnetic echo energy in return; beam scanning means for causing said bearn to scan through a predetennined path in space; a radar pulse receiver comnected to receive said echo energy from said bearn radiating means, the gain of said receiver being controllable over the duration of, and in response to gain control pulses, und said receiver having a video output; storage means scanned in synchronism With said beam radiating means anal adaptecl to accept and stete said Video output, said storage means being 0f a type relatlvely insensitive to any portion of said Video output corresponding to -a moving target echo and therefore not recurring in lndentioal position thronghout successive scans; and pulse translating means connected to saicl storage means and adapted to trnnsrnit gain control pulses to said receiver in accordance with signals stored in said storage means.

5. A radar system including at least one scanning antenna for scanning a beam of electromagnetio energy through a volume of space and being adapted to present ecl1o signals corresponding to objects within the scanned volume of space and to provide at least some discrirnlnation against echo signals corresponding to stationary objects, cornprising the combination of: pulsed radar transmitting means; radar receiving means for detecting portions of the energy of said transrnitting n1eans echoed by moving and stationary objects, said receiving means inclnding pulse responsive means Within the signal channel o1" said receiving, means for controlling the gain applicable to discrcte ones of said echo signals in response to contempornneous pulsed gain control signals; storage means comnected to tl1e output of said receiving means to store seid target echo signals, said storage mean being of a type which stores signal intensities as a function 0f the individual tin1e integral for euch o seid target echo signals; and means for generating seid pulsed gain control signals CO1- responding to and proportional to the stored intensities of the signals representative of said echo signals, thereby to elfect substantially greater pulsed gain-reducing feedback in tirne relation with said echo signals corresponding t0 stationary objects than for those corresponding to moving objects.

6. The invention set forth in claim 5 further defined in that said scanning antenna includes means for developing a control signal continuously representative of its instan- E2 taneous scanning position and said storage rneans is of the cathode-ray type and includes storage scanning n1eans responsive to said control signal thereby etlecting scanning of sald storage surface in synchronism with the scan of seid antenna.

7. In a pulsed radar system including a directional antenna With means for scanrn'ng in angle, transmitting means for supplying pulses of electrornagnetic energy to said antenna, and cathode ray indicating rneans having range and angle sweeps; the combination comprising: a radar receiver having an input connected to receive echo signals from said antenna, said receiver having an output and at least one controllable stage responsive to gainreducing pulses whereby the gain of said receiver may be reduced during and in proportion to tl1e =amplitude of seid gain-redncing pulses; cathode ray storage means having a writing input for signals to be stored, and a reading output for reading stored signals; means for supplying signals fron1 said receiver output to said writing input, said rneans belng connected so as to pass only signals above a predetern1ined thresholcl level; rneans synchronized With said indieating means sweeps, said antenna angular scan, and seid transrnitted ulses for causing angle and range sweeps of said oathode ray storage rneans in synchronisrn Witl1 corresponding range and angle sweeps of said indicating means; rneans for predeterminlng tl1e learning tirne constant of seid storage means whereby more than one successive occurrence of said signals from said receiver out put at said writing input at a particular range and angle position is required to effect storage and thereoy to produce corresponding signals at seid reading output; and means responsive to signals at said reading output to produce and apply said gnin-reducing pulses to said receiver controllable stage.

3. The invention set fortl1 in claim 7 further defined in that said cathocle ray storage means includes 21 scanned charge pattern storage surface, and said means for predetennining the learning tirne constant of said storage rneans inclucles the charge pattern storage surface itself with its inherent time and intensity integration characteristic.

9. A system of the character described comprising the comblnation of: a radar pulse receiver the gain of which is controllable over the duration of, and in response to gain control pulses, said receiver having a video output; a selection circuit operating on said video output t0 produce selected signals by passing only Signal arnplitudes within saicl Video output in excess of a predetermined value; storage means for storing said selected signals; storage control means interposed between said selection circuit and seid storage means for controlling the response rate of said storage means; and pulse translating means sonnected to said storage rneans to generate and transmit gain control pulses corresponding in occurrence and duration to said stored selected signals to said receiver.

1%. The invention set forth in clairn 9 in which said storage control means includes means to control the learning tin1e of seid storage means in a manner such that a plurality of occurrences of any given signal at the same range and angle on successive scans are necessary for storage of said signal.

References Citecl by the Examina UNITED STATES PATENTS 2,956274 10/60 Smythe 3437.7 2986673 5/61 Schagen et al 3437.7 2,989742 6/61 Johnson et a1. 3437.7 305l947 8/62 Engel 3437.7

CHESTER L. IUSTUS, Primary Examina).

FREDERICK M STRADER, Examz'zze1x 

5. A RADAR SYSTEM INCLUDING AT LEAST ONE SCANNING ANTENNA FOR SCANNING A BEAM OF ELECTROMAGNETIC ENERGY THROUGH A VOLUME OF SPACE AND BEING ADAPTED TO PRESENT ECHO SIGNALS CORRESPONDING TO OBJECTS WITHIN THE SCANNED VOLUME OF SPACE AND TO PROVIDE AT LEAST SOME DISCRIMINATION AGAINST ECHO SIGNALS CORRESPONDING TO STATIONARY OBJECTS, COMPRISING THE COMBINATION OF: PULSED RADAR TRANSMITTING MEANS; RADAR RECEIVING MEANS FOR DETECTING PORTIONS OF THE ENERGY OF SAID TRANSMITTING MEANS ECHOED BY MOVING AND STATIONARY OBJECTS, SAID RECEIVING MEANS INCLUDING PULSE RESPONSIVE MEANS WITHIN THE SIGNAL CHANNEL OF SAID RECEIVING MEANS FOR CONTROLLING THE GAIN APPLICABLE TO DISCRETE ONES OF SAID ECHO SIGNALS IN RESPONSE TO CONTEMPORANEOUS PULSED GAIN CONTROL SIGNALS; STORAGE MEANS CONNECTED TO THE OUTPUT OF SAID RECEIVING MEANS TO STORE SAID TARGET ECHO SIGNALS, SAID STORAGE MEAN BEING OF A TYPE WHICH STORES SIGNAL INTENSITIES AS A FUNCTION OF THE INDIVIDUAL TIME INTEGRAL FOR EACH OF SAID TARGET ECHO SIGNALS; AND MEANS FOR GENERATING SAID PULSED GAIN CONTROL SIGNALS COR- 